1. Field of the Invention
The present invention relates to an optical control element, and particularly to an optical control element, such as an optical modulator or an optical switch, which includes a substrate having an electro-optical effect, an optical waveguide formed on the substrate, and a control electrode for controlling light waves propagating through the optical waveguide.
2. Description of Related Art
In an optical communication field or an optical measurement field, an optical control element such as an optical modulator or an optical switch is frequently used in which an optical waveguide and a control electrode for controlling light waves propagating through the optical waveguide are provided on a substrate having an electro-optical effect.
In addition, in order to detect an operation state of the optical control element, for example, there has been proposed a method in which radiation mode light emitted from an optical modulator is reflected by grooves provided in a substrate and is monitored by a photodetector as disclosed in Japanese Laid-open Patent Publication No. 4-24610, a method in which higher mode light such as radiation mode light is derived outward from a substrate by using a monitoring optical waveguide as disclosed in Japanese Laid-open Patent Publication No. 2006-301612 or Japanese Laid-open Patent Publication No. 2008-46589, or the like.
In the optical control element, fundamental mode light which is signal light and higher mode light such as radiation mode light of an optical modulator or OFF light of an optical switch are required to be appropriately separated, and, it is essential for the separated higher mode light not to be incident again to the optical waveguide through which the fundamental mode light propagates, or for the higher mode light not to be incident more than necessary, as stray light, to monitoring means for monitoring an operation state of the optical control element as described above.
For this reason, a shielding means is provided in a region where the higher mode light such as radiation mode light propagates. However, a method, in which a depressed portion, such as a groove, is formed in a substrate as the shielding means and a light absorption material is buried in the depressed portion, complicates a manufacturing process, and causes the mechanical strength of the substrate to be reduced in a thin substrate. In addition, a method is disclosed in which a high refractive index region is formed in a substrate or a light absorption material film is formed on a substrate surface; however, many constituent elements such as an optical waveguide and a control electrode are required to be incorporated into the substrate surface. Therefore, if an installation region of the shielding means is sufficiently secured in order to exclude unnecessary higher mode light in advance, a degree of freedom of design of the optical control element is restricted, and this impedes miniaturization or the like of the optical control element.